1. Field of the Invention
The present invention relates to an engine exhaust pipe, an engine exhaust system, and a saddle riding type vehicle including an exhaust system and exhaust pipe, and a method of manufacturing and mounting an engine exhaust pipe, and more particularly relates to a technique of preventing exhaust gas from leaking from an exhaust pipe, without lowering productivity of the exhaust pipe.
2. Description of the Related Art
With a saddle riding type vehicle represented by a two-wheeled motor vehicle and the like, an engine exhaust pipe may be visible as part of its outward appearance. Therefore, the outward appearance may be improved by applying surface treatment such as chrome plating to the exhaust pipe. In this case, the exhaust pipe can be made into a hollow double structure having an outer pipe and an inner pipe, to prevent the surface of the outer pipe from discoloring due to the temperature of exhaust gas, thereby to maintain the outward appearance (see Japanese Unexamined Patent Publication H11-257072 and Japanese Unexamined Patent Publication No. 2002-332838, for example).
Japanese Unexamined Patent Publication No. 2002-332838 describes that, when applying chrome plating to the exhaust pipe in the form of a double pipe, the outer pipe and inner pipe are welded to seal a space between the outer pipe and inner pipe.
However, the conventional example with such construction has the following problem.
That is, with the conventional apparatus, when surface treatment is carried out in a state where the space between the outer pipe and inner pipe is not sealed, a treating liquid for the surface treatment such as plating liquid will flow into the space between the outer pipe and inner pipe. In this case, it takes time to drain the treating liquid from this space, which will lower productivity. The treating liquid remaining in this space will cause corrosion of the exhaust pipe.
Therefore, it is known to form drain holes in a peripheral surface of the outer pipe. This measure allows the treating liquid for the surface treatment to be drained promptly, and the surface treatment to be carried out efficiently. With these drain holes, instead of being limited to the surface treatment, it is possible to drain a liquid promptly, such as water filled and frozen in the space between the outer pipe and inner pipe at a time of bending a multiple pipe. After attaching the exhaust pipe to a saddle riding type vehicle, ambient air can flow into and out of the space between the outer pipe and inner pipe through the drain holes, thereby cooling the exhaust pipe effectively. When the drain holes are formed in a peripheral surface of the inner pipe, the space between the outer pipe and inner pipe will be added to the capacity of the exhaust passage, which renders engine output characteristics different from what has been intended. It is therefore preferable to form the drain holes in a peripheral surface of the outer pipe.
When an exhaust gas detector such as an oxygen sensor, or a branch piping member such as piping which introduces secondary air into the exhaust pipe (hereinafter referred to as “sensor or the like”), is attached to the exhaust pipe, through-holes are formed in the outer pipe and inner pipe, respectively, and the sensor or the like is inserted from outside the outer pipe into the inner pipe. At this time, in the case of the oxygen sensor, by welding to the outer pipe a boss for attaching the oxygen sensor, for example, the sensor or the like can be made to penetrate the outer pipe in a gas-tight state. However, it is difficult to make the sensor or the like penetrate the inner pipe in a gas-tight state. There is a possibility that exhaust gas flowing within the inner pipe flows into the space between the inner pipe and outer pipe. Thus, there is a possibility that exhaust gas leaks from the exhaust pipe to the atmosphere when drain holes are formed in the outer pipe as in the prior art. When attaching the sensor or the like to the exhaust pipe, exhaust holes cannot be formed as in the prior art. As a result, it becomes difficult to drain the above-noted treating liquid promptly, and hence an inconvenience of lowering productivity of the exhaust pipe occurs.